1. Field of the Invention
The present invention relates to improved reaction chambers used in Chemical Vapor Deposition (CVD) systems and, more particularly, to a method for controlling reactant gas flow in reaction chambers for use in epitaxial deposition systems for processing wafers on a one at a time basis.
2. Description of the Prior Art
Chemical Vapor Deposition (CVD) is the formation of a stable compound on a heated substrate or wafer by the thermal reaction or decomposition of certain gaseous compounds. Epitaxial growth is a specific type of CVD which requires that the crystal structure of the wafer be continued through the deposited layer.
The basic components of any CVD system include a reaction chamber which houses the wafer(s) to be processed, a gas control section, a timing and sequence control section, a heat source and an effluent handling component. The reaction chamber provides a controlled environment for the safe deposition of stable compounds. The chamber boundary may be quartz, stainless steel, aluminum or even a blanket of a non-reacting gas (such as nitrogen). Commercial epitaxial deposition reaction chambers may be one of three types, depending primarily upon the gas flow. In horizontal systems, the gas flows horizontally in one end of the reaction chamber, across the wafers, and out the other end. In vertical systems, the gas flows vertically toward the wafers from the top and the susceptor is normally rotated to provide more uniform temperature and gas distributions. In a cylindrical or barrel reactor system, the gas flows vertically into the chamber from the top and passes over the wafers on a rotating susceptor.
Heating in a cold wall CVD system is accomplished by radio frequency (RF) energy, radiation energy in the ultraviolet (UV) or infrared (IR) bands or resistance heating. In an RF heated susceptor, the energy in an RF coil is coupled into a silicon carbide coated carbon susceptor and the wafers are heated by conduction. Radiant UV or IR heating is accomplished by the use of high intensity lamps to heat the wafers and their holders. The chamber walls must be cooled to prevent a large temperature rise of the reaction chamber. In an epitaxial deposition system a carefully controlled environment is needed for the epitaxial deposition to take place.
The various gases used in an epitaxial reaction chamber include a nonreactive purge gas used at the start and end of each deposition if the reaction chamber is opened to the atmosphere after every run. The nonreactive purge gas, usually nitrogen, flushes unwanted gases from the reaction chamber. A carrier gas is used before, during and after the actual growth cycle. It is mixed with the gases responsible for etching, growth, or doping the wafer. Hydrogen is most often used as a carrier gas, although helium is sometimes employed. Etching gases may be used prior to the actual epitaxial deposition to remove a thin layer of silicon from the surface of the wafer together with any foreign matter or crystal damage that may be present. The etching prepares atomic sites for nucleating or initiating the epitaxial deposition process. The source gases for epitaxial depositions include Silane (SiH.sub.4), Dichlorosilane (SiH.sub.2 Cl.sub.2), Trichlorosilane (SiHCl.sub.3) and Silicon tetrachloride (SiCl.sub.4). The dopant gases normally used in epitaxial deposition include Arsine (AsH.sub.3), Phosphine (PH.sub.3), and Diborane (B.sub.2 H.sub.6). The etching gas is commonly HCl.
The problems inherent in all prior art systems of CVD, particularly in the epitaxial deposition, include: non-uniform deposition on the surface of the wafer to be processed, the presence of contaminants in the reaction chamber prior to processing, wall deposits formed on the interior walls of the reactor chamber; deposition of the reactant chemicals on the heated susceptor and its support structure, inefficient gas flow characteristics, slow processing times and non-uniform depositions due to uncontrolled gas velocity profiles or gas density profiles.
These problems become even more important with the modern trend away from batch processing systems toward single wafer or one substrate at a time processes. In a single wafer processing system, the same volume of gas normally flowing through a reaction chamber with many wafers to be processed cannot be used since too much reactant gas will be consumed for one wafer. The cycle times to process a batch are far too long for single wafer processing. A single wafer process requires a more rapid deposition rate to minimize the cycle time. In single wafer processing, the deposits from reaction by-products build up far more rapidly on a per wafer basis than in batch processing. Customers are increasingly demanding reduced particulate contamination.